Mechanical characterization of bottom hole assembly in air drilling engineering

To study the mechanism and control of well deviation in air drilling, a mechanical model of bottom hole assembly (BHA) with double stabilizer is established by using the beam-column method. The mathematical model related to side force of BHA and deviating force of strata are derived by analyzing the characteristics of gravel strata in the practical engineering. The effects of weight on bit (WOB), expansion ratio of borehole diameter (ERBD), angle of well inclination, diameter of drill collar and strata bending moment on side force of BHA are explored. Then, the superposition of bit side force and deviating force of strata is obtained. The results display that side force of BHA is obviously influenced by ERBD, angle of well inclination, diameter of drill collar and strata bending moment; and the deviation force and azimuth force of gravel strata are significantly affected by comprehensive cutting anisotropy index, angle of well inclination and dip angle of strata. Finally, the variation tendency of inclination angle and azimuth angle in field measurement is consistent with the theoretical and numerical researches. The present work can provide theoretical guidance to control the well deviation in the practical air drilling.